Gliomas, the most common adult-onset neurological neoplasm, encompass a family of primary central nervous system tumors including glioblastoma, astrocytoma, oligodendroglioma, and ependymoma, along with the juvenile onset neoplasms such as juvenile pilocystic astrocytoma.
Malignant gliomas are typically characterized by over-expression of growth factors/tumor associated antigens believed to significantly contribute to the unchecked growth of such tumors. Various malignant gliomas, such as glioblastomas, exhibit epidermal growth factor receptor (EGFR) overexpression leading to increased aggressiveness and poor prognosis. Malignant gliomas may also display over-expression of platelet-derived growth factor receptor, a phenomenon which has also been correlated with increased malignancy and poor prognosis.
Malignant gliomas (WHO grade III-IV), the most common type of primary brain tumors, are aggressive, highly invasive, and neurologically destructive tumors, which are among the deadliest of all human cancers. Of the estimated 17,000 new brain tumors diagnosed each year in the United States, about half are malignant gliomas. Malignant glioma cells produce very invasive brain tumors with infiltration of both white and gray matter. At the time of diagnosis, microscopic extension through much of the neural axis by malignant glioma is the rule. Such extension by motile invading cells underlies the incurability by surgery of most gliomas, even when they appear small and restricted in nature.
Glioblastoma (GBM) (WHO grade IV), the most serious form of malignant glioma, are extremely aggressive brain tumors which generally arise in the upper brain (cerebrum), but which may also occur elsewhere in the central nervous system, such as in the spinal cord, cerebellum, brain stem, or optic chiasm. Low-grade gliomas (WHO grade I-II), which include astrocytomas, oligodendrogliomas, and pilocytic astrocytomas, account for 25% of all primary brain tumors, and over time most of these low-grade tumors dedifferentiate into more malignant gliomas. Diffuse astrocytomas are predominantly located in the cerebral hemispheres of adults and have an inherent tendency to progress to anaplastic astrocytoma (WHO grade III) and (secondary) glioblastoma (WHO grade IV). The majority of glioblastomas develop de novo (primary glioblastomas), without an identifiable less-malignant precursor lesion.
Glioblastoma is the most common and aggressive primary brain tumor in adults. Its prognosis remains extremely poor, despite multimodal treatment by surgery, radiotherapy and chemotherapy (Wen et al, N Engl J Med 2008, 359: 492-507). The median survival of patients with glioblastomas is only 12-15 months. When these tumors recur, conventional salvage therapies produce minimal benefit, with only 8-15% of patients alive and free from progression at 6 months (6M-PFS). These tumors are now well characterized at the transcriptome and genome levels. Several studies have demonstrated that a combination of these two molecular levels may be advantageous for determining robust signatures and clinically relevant molecular classifiers of glioblastoma (de Tayrac et al, Genes Chromosomes Cancer 2009, 48: 55-68; Nigro et al, Cancer Res, 2005, 65: 1678-1686).
The Gold standard for the diagnostics of gliomas is by histopathological and immunohistochemical examination by a neuropathologist according to the WHO classification of central nervous systems (D. N. Louis, H. Ohgaki, O. D. Wiestler and W. K. Cavenee, ed., International Agency for Research on Cancer (IARC) Press, Lyon, 2007). The World Health Organization (WHO) classification divides gliomas into three main subgroups: astrocytomas, oligodendrogliomas, and mixed gliomas (oligoastrocytomas). It further distinguishes between four malignancy grades (WHO grades I-IV). Because phenotypic heterogeneity within these tumors is quite frequent, the histopathologic examination yields differing results, even when performed by experienced neuropathologists. Especially the differentiation between glioblastoma multiforme (WHO grade IV) and anaplastic glioma (WHO grade III) with either oligodendroglial, astrocytic or both features could be very difficult. Moreover, the clinical outcome is often not predictable, which may reflect biological heterogeneity within each of the tumor groups. Research during the last decades has thus been focused on a more accurate characterization of these tumors including the identification of new prognostic markers in order to supply and complement histology-based classification.
A few clinically-relevant biomarkers have been identified so far in glioblastoma. The somatic mutation affecting amino acid 132 of the isocitrate dehydrogenase 1 (IDH1) protein is an independent prognostic biomarker associated with better clinical outcome in gliomas, including glioblastoma (Ichimura et al, Neuro Oncol, 2009, 11: 341-347; Yan et al, N Engl J Med, 2009, 360: 765-773), but this mutation is rare in glioblastoma (around 6%) and concerns almost exclusively secondary glioblastomas (Sanson et al, J Clin Oncol, 2009, 274150-4154). Changes in promoter DNA methylation pattern of genes involved in key biological pathways have been reported in glioblastoma. For instance, the retinoblastoma (RB), the PI3K and p53 pathways are affected by CpG island promoter hypermethylation (RB, CDKN2A, PTEN, TP53) (Watanabe et al, J Neuropathol Exp Neurol, 2001, 60: 1181-1189; Nakamura et al., Lab Invest, 2001, 81: 77-82; Costello et al, Cancer Res, 1996, 56: 2405-2410; Bello et al, Cancer Genet Cytogenet, 2006, 164: 172-173; Amatya et al, Acta Neuropathol, 2005, 110: 178-184).
In some cancers, chromosomal rearrangements (created when chromosomes break and then recombine incorrectly) may create fusion genes where parts of two genes combine together so that the latter part of a certain gene falls under the control of the promoter of another. This can lead to either over-activation of a certain proto-oncogene (a gene whose over-expression contributes to carcinogenesis; these are usually involved in cellular growth and proliferation) or abolishment of the function of a tumor suppressor gene. The most famous example is the fusion of bcr and c-abl in chronic myelogenic leukemia (CML) and less frequently in acute lymphoblastic leukemia (ALL) which is created from the fusion of parts of chromosomes 9 and 22. This fusion causes an over expression of the proto-oncogene c-Abl which in turn leads to an acceleration of cell proliferation. These fusion genes create lucrative therapy targets as they are specific to tumor cells and therefore, understanding their function is imperative to creating new therapies (reviewed in: Yeung and Hughes (2012) Therapeutic targeting of BCR-ABL: prognostic markers of response and resistance mechanism in chronic myeloid leukemia. Crit. Rev. Oncog. 17: 17-30; Wong S F and Mirshahidi H (2011) Use of tyrosine kinase inhibitors for chronic myeloid leukemia: management of patients and practical applications for pharmacy practitioners. Ann. Pharmacother. 45: 787-797.).
Multifocal glioblastomas are especially aggressive cases of glioblastoma (GBM) where multiple tumors are found in the brain of the patient. These cancers form around 10% of diagnosed GBM cases and their cause is not fully understood. It can be assumed that these tumors have an especially invasive nature with an enhanced ability to transplant in other regions of the brain. Treatment of brain cancers is not easy or very effective; remission rates remain high. Surgery is hampered by the fact that GBM cells are highly infiltrative and the surgeon doesn't have the option of removing normal tissue around the tumor. In addition, populations of GBM cells are radio- and chemoresistant making radio- and chemotherapy not completely efficient. Furthermore, treatment success is increasingly found to depend on the proper diagnosis and molecular classification.
Accordingly, there is a great need for better molecular markers, in particular for the diagnosis and treatment of GBM.
Roundabout (Robo) 2 is a single-pass transmembrane receptor which functionally resembles receptor tyrosine kinases (RTK). It is a member of a family of four such receptors (Robo1-4) that bind to their ligands, members of the Slit family. This binding is mediated by the first two Ig-like domains in the extracellular part of the Robo receptor. The Robo-Slit signaling has been reported to be important for axon guidance in neurons and for migration of glial cells and thus can be thought to be important for migration and invasion, two features of aggressive cancers. However, the Robo-Slit pathway is now found to play a role in other cellular phenomena. For example, Robo receptors are reported to be involved in the proper differentiation and localization of neuronal progenitor cells. Deletion of Robo2 inhibited the differentiation of these cells into glial cells and their proper localization in the subventricular zone (SVZ). Robo signaling was also found to be important for the maintenance of the stem cell population in the intestinal crypt.
Roundabout 2 (Robo2) is a member of the Roundabout family of receptors which includes Robo1-4. The focus of these receptors has been on their role in axonal guidance and adhesion; however, their role in stem cell maintenance and differentiation is emerging (Zhou, W J et al. Nature, 2013, 501: 107-113; Borrell, V et al. Neuron, 2012, 76: 338-352). Therefore, it is expected that Slit-Robo aberrant signaling would play a role in tumorigenesis. The role of Slit-Robo signaling has been recently reported in pancreatic cancer (Tang et al., Carcinogenesis, 2014, [Epub ahead of print]), ovarian cancer (Dickinson et al., PLoS One, 2011, 6(11):e27792), melanoma (Denk et al, Int J Mol Med, 2011, 28:721-6), squamous cell carcinoma (Bauer et al., Carcinogenesis, 2011, 32:935-43), hepatocellular carcinoma (Avci, M E et al., BMC Cancer, 2008, 29: 392-403), colorectal carcinoma (Zhou W J et al., Cell Res., 2011, 21: 609-626), and indeed in epithelial tumors in general (Ballard and Hinck, Adv Cancer Res, 2012, 114:187-235). Furthermore, the pathway has been associated with angiogenesis (Ballard and Hinck, Adv Cancer Res, 2012, 114:187-235; Yang, X M et al., Biochem Biophys Res Commun, 2010, 28: 396).
KIAA0368 (otherwise known as ECM29) is a protein made up of several HEAT domains. It's described in the literature as an adaptor protein, mostly involved in binding to the 26S proteasome and aiding in its proper assembly and transport by linking it to motor proteins and several cellular compartments such as the ER and Golgi apparatus (Gorbea C et al., J Biol. Chem., 2010, 285: 31616-31633).